Metal Phosphides Derived from Hydrotalcite Precursors toward the Selective Hydrogenation of Phenylacetylene

We report a new synthetic strategy for the fabrication of several supported nickel phosphides (Ni12P5, Ni2P, and NiP2) with particle size ranging from 5 to 15 nm via a two-step procedure: preparation of supported Ni particles from layered double hydroxide precursors, followed by a further reaction with a certain amount of red phosphorus. The selective hydrogenation of phenylacetylene over these metal phosphides was evaluated, and the as-prepared Ni2P/Al2O3 catalyst shows a much higher selectivity to styrene (up to 88.2%) than Ni12P5/Al2O3 (48.0%), NiP2/Al2O3 (65.9%), and Ni/Al2O3 (0.7%) catalysts. EXAFS and in situ IR measurements reveal that the incorporation of P increases the bond length of Ni-Ni, which imposes a key influence on the adsorption state of alkene intermediates: as the Ni-Ni bond length extends to 0.264 nm, the alkene intermediate undergoes di-pi(C=C) adsorption, facilitating its desorption and the resulting enhanced selectivity. Moreover, electron transfer occurs from Ni to P, as confirmed by EXAFS, XPS, and in situ CO-IR experiment, in which the positively charged Ni reduces the desorption energy of alkene and thus improves the reaction selectivity.